Strain monitoring system and apparatus

ABSTRACT

A system for monitoring strain as an indicator of biological conditions, such as spinal fusion, glucose levels, spinal loading, and heart rate. The system includes an inter-digitated capacitor sensor, and RF transmitter, and an associated antenna, all of which are microminiature or microscopic in size and can be implanted in a biological host such as a human or animal. An inductively coupled power supply is also employed to avoid the need for implantation of chemical batteries. Power is provided to the sensor and transmitter, and data is transmitted from the sensor, when an external receiving device, such as a handheld RF ID type receiver, is placed proximate the location of the implanted sensor, transmitter and inductively coupled power supply. The implanted sensor, transmitter and inductively coupled power supply can be left in place permanently or removed when desired.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/620,973 filed on Jan. 8, 2007, now U.S. Pat. No. 8,070,695 issued onDec. 6, 2011 and incorporated herein by reference in its entirety, whichis a 35 U.S.C. §111(a) continuation of PCT international applicationserial number PCT/US2005/024340, filed on Jul. 8, 2005 and incorporatedherein by reference in its entirety, which is a nonprovisional of U.S.provisional patent application Ser. No. 60/586,593 filed on Jul. 8, 2004and incorporated herein by reference in its entirety. Priority isclaimed to each of the foregoing applications.

This application is related to PCT International Publication No. WO2006/010037 A2, published on Jan. 29, 2006, incorporated herein byreference in its entirety.

This application is also related to U.S. patent application Ser. No.11/620,980 filed on Jan. 8, 2007, now U.S. Pat. No. 8,066,650 issued onNov. 29, 2011 and incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

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NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. §1.14.

A portion of the material in this patent document is also subject toprotection under the maskwork registration laws of the United States andof other countries. The owner of the maskwork rights has no objection tothe facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all maskwork rights whatsoever. The maskwork owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. §1.14.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to monitoring strain, and moreparticularly to using strain monitoring as an indicator of medicalconditions including monitoring the progress of spinal fusion,monitoring glucose levels, measuring spinal loading, and monitoringheart rate.

2. Description of Related Art

Lumbar fusion is one of the fastest growing areas of orthopedic surgery.

The most common indication for surgical intervention is pain in thelower back. Although many devices have been designed to minimize theincidence of work-related back injury, as a society we still participatein many activities that lead to back injury. Most frequently,inappropriate lifting of objects, pulling or lifting objects fromawkward angles, and fatigue lead to injury of the back muscles. If thelevel of injury is severe enough, the muscles and ligaments of thelumbar spine cannot withstand the load applied, and the intervertebraldisc will become herniated from the anterior side of the spine. This isoften called a herniated or ruptured disc. In addition, the vertebrae ofthe spine articulate with each other through the transverse and spinousprocesses located on the posterior aspect of the vertebrae. In betweenthe processes, there are small pads of cartilage that can become damagedwith a back injury. Both herniated discs and the processes can causechronic pain and loss of function in the spine. Pain results indebilitation and prevents the patient from enjoying ordinary dailyactivities.

To eliminate the pain, a lumbar fusion is performed wherein an incisionis made over the lumbar region of the spine and metal bracing is appliedbilaterally to the posterior of the vertebrae. This bracing providesinitial mechanical stiffness until bone growth, stimulated by a bonegrowth factor, encapsulates the metal bracing and eliminates motionbetween the two lumbar vertebrae. There are many choices for the metalbracing, called spinal instrumentation, which can be used to create theinitial fixation. In general, a pedicle screw is screwed from theposterior through the pedicle bony bridge of the vertebrae and into thewall the vertebral body. This procedure is repeated for the neighboringvertebrae and bilaterally on the opposite side of the posterior spine.Once all four pedicle screws are in place, a rod or plate is placed overposts on two of the pedicle screws. The rod or plate is then held downwith locking nuts that screw onto the posts. A slurry of bone and bonegrowth factor is applied over the spinal instrumentation and vertebrae,and the incision is closed.

After lumbar fusion surgery, rehabilitation takes several months. Thepatient is immobilized with a brace that extends from beneath the armsto midline of the hips and is instructed not to perform any strenuousphysical activity. No lifting, driving, running or bending at the waistis allowed. Any kind of activity that involves impact is alsoprohibited, such as roller coasters. The patient must wear the braceuntil fusion is visible on an x-ray radiograph. Depending on the age ofthe patient, this can be anywhere from four months to a year aftersurgery. Because of this extended period of immobility, the muscles ofthe spine and abdomen atrophy from disuse. The brace also contributes tostress shielding, meaning the brace is carrying some of the spinal load,resulting in an inferior strength lumbar fusion.

The problem with the foregoing treatment approach, is that fusion occursmuch sooner than is predicted by radiographs. For example, a solidfusion could occur as early as eight weeks (two months) after surgery.However, the bone that initially grows around the spinal instrumentationis trabecular bone, and although it is strong and dense, it is notradiographically opaque. Thus, it cannot be seen on an x-ray until ithas been infused with minerals, such as calcium.

There are several methods for measuring the movement or strain in thehuman spine, including those that involve collecting an electronicsignal and transmitting it to an external receiver. For example, U.S.Pat. No. 6,433,629 teaches using a Wheatstone bridge and a timingcircuit to measure the displacement (strain) in an orthopedic implant.In addition, the device does not use an internal power source. Instead,a magnetic coil brought in close proximity to the Wheatstone bridgeprovides power to the circuitry and activates the circuitry for theduration of the measurement.

In U.S. Pat. No. 5,935,086, the relative angles between two or morejoint are measured and a force transducer is used to simultaneouslymeasure the applied force in the joint of an artificial knee. This issimilar to U.S. Pat. No. 5,995,879, which also measures the anglebetween two freely movable points to determine the orientation of asecond spinal vertebrae relative to a first vertebrae.

U.S. Pat. No. 6,432,050 uses audible acoustic feedback to monitor an invivo sensor or device. By applying an acoustic query to the implanteddevice, the operator can audibly determine if the device is functioningproperly. This has wide reaching applications, from heart surgerystents, to intervertebral disc implants.

In U.S. Pat. No. 6,223,138, a Wheatstone bridge is used to measurestrain displacement, but the signal is amplified and added it to acarrier frequency. By adding the signal to a secondary frequency, lossof a small signal in the background noise is avoided.

Published U.S. patent application number US200210050174 A1 also usesstrain gages in a Wheatstone bridge, the device has been adapted tosuccessfully measure strains on the micron scale.

Published U.S. patent application number US2004/0011137 A1 also providesinformation concerning the current state of the art.

Each of the foregoing U.S. patents and published patent applications isincorporated herein by reference in its entirety.

Notwithstanding the foregoing approaches to measuring strain, the onsetof spinal fusion after lumbar surgery continues to be difficult todetermine, and patients are frequently fitted with a spinal brace forthree to six months after surgery even though the implant providesinternal fixation in a much shorter period of time. If a new methodcould be developed that could detect a solid fusion without the need forradiographic verification, the amount of time patients would need to bein a brace could be cut by 50% or more.

Similarly, there is a need for new approaches to monitoring strain inother parts of the body and for monitoring other medical conductions.The present invention satisfies those needs and advances the state ofthe art.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises a system and apparatus that satisfiesthe foregoing needs through the use of four main features: strainsensing, integrated microfabricated circuitry, RF signal transmission,and data collection. According to an aspect of the invention, the sensorcomprises an inter-digitated capacitor. Another aspect of the inventionis the microminiaturization of a strain sensing system. Using thetechniques of the present invention, a strain sensing system can bemicroscopic in size. The resultant miniaturization allows the system tobe incorporated or integrated into an implant or other device. Anotheraspect of the invention is the elimination of the need for an internalbattery power supply or external leads connecting the system to anexternal power supply. This is accomplished through the use of aninductively coupled power supply.

According to another aspect of the invention, strain monitoring is usedas an indicator of medical conditions including monitoring the progressof spinal fusion, monitoring glucose levels, measuring spinal loading,and monitoring heart rate.

By way of example, and not of limitation, for monitoring spinal fusion,the inventive strain sensor system can be bonded to the implant, whichwill be load sharing with the bone. Thus, as the spine heals, theimplant strain will diminish. In this embodiment, the inventioncomprises an implantable strain transduction system for humans fordetermining when fusion has occurred.

Accordingly, the present invention generally comprises an implantableinter-digitated capacitor based strain sensor system that can produce areliable, reproducible signal that will indicate via a radio telemetrysignal when strain has changed.

In one exemplary embodiment, the invention contains a strain sensor thatwill accurately measure low levels of strain and transmit the data usingan RF transmitter and associated antennal. In another embodiment, theimplantable portion of the system is inductively powered by an externalelectromagnetic power source to avoid the complications of implantingbatteries within humans. Otherwise, batteries can be mountedsubcutaneously and later removed.

In another embodiment, an apparatus for sensing strain comprises aninter-digitated capacitor sensor, a transmitter, and an antenna, whereinthe sensor, transmitter, and antenna are adapted for implantation in abiological host.

In another embodiment, a system for sensing strain comprises aninter-digitated capacitor sensor, a transmitter, an antenna, and areceiver, wherein the sensor, transmitter, and antenna are adapted forimplantation in a biological host, and wherein the receiver is anon-implantable remotely operated device.

In a further embodiment, a system for sensing strain comprises aninter-digitated capacitor sensor, a transmitter, an antenna, aninductively coupled power supply, and a receiver, wherein the sensor,transmitter, antenna, and power supply are adapted for implantation in abiological host, and wherein the receiver is a non-implantable remotelyoperated device.

In still another embodiment, an apparatus for sensing strain comprisesan inter-digitated capacitor sensor, a transmitter, and an antenna,wherein the sensor, transmitter, and antenna are adapted forimplantation in a biological host, wherein the sensor is adapted formounting to a spinal plate and configured to produce a signalrepresentative of strain in said spinal plate, and wherein thetransmitter is configured for transmitting signals representative ofstrain.

Another embodiment of the invention is a system for sensing straincomprising an inter-digitated capacitor sensor, a transmitter, anantenna, and a receiver, wherein the sensor, transmitter, and antennaare adapted for implantation in a biological host, wherein the sensor isadapted for mounting to a spinal plate and configured to produce asignal representative of strain in said spinal plate, wherein thetransmitter is configured for transmitting signals representative ofstrain, and wherein the receiver is a non-implantable remotely operateddevice.

A further embodiment of the invention is a system for sensing straincomprising an inter-digitated capacitor sensor, a transmitter, anantenna, an inductively coupled power supply, and a receiver, whereinthe sensor, transmitter, antenna, and power supply are adapted forimplantation in a biological host, wherein the sensor is adapted formounting to a spinal plate and configured to produce signalsrepresentative of strain in the spinal plate, wherein the transmitter isconfigured for transmitting signals representative of strain, andwherein the receiver comprises a non-implantable remotely operateddevice.

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a functional block diagram of an embodiment of a strainsensing system according to the invention.

FIG. 2 is a functional block diagram of an embodiment of the RFtransmitter subsystem in the system shown in FIG. 1.

FIG. 3 is a graph illustrating waveform response of the voltagecontrolled oscillator shown in FIG. 2 to an AC modulated signal.

FIG. 4 is a cross-sectional schematic view of an embodiment of amicrostrip antenna according to the invention.

FIG. 5 is a top plan schematic view of an alternative embodiment of themicrostrip antenna shown in FIG. 4.

FIG. 6 is a cross-sectional schematic view of the microstrip antennashown in FIG. 5.

FIG. 7A through FIG. 7R is a flow diagram showing an embodiment of aprocess for fabricating the microstrip antenna shown in FIG. 5 and FIG.6.

FIG. 8 is a schematic diagram of an embodiment of an inductive powersupply subsystem according to the invention.

FIG. 9A is a schematic view of an area variation motion capacitor strainsensor employed in the present invention.

FIG. 9B is a perspective view of the structure of an inter-digitatedarea variation motion capacitor strain sensor according to the presentinvention.

FIG. 10A is a schematic diagram of a capacitance bridge employing aninter-digitated capacitor strain sensor according to the invention.

FIG. 10B is a schematic diagram of an equivalent circuit to the circuitshown in FIG. 10A.

FIG. 11 is a schematic diagram of a differential amplifier employed inthe inter-digitated capacitor strain sensor according to the presentinvention.

FIG. 12A through FIG. 12WW is a flow diagram showing an embodiment of aprocess for fabricating an inter-digitated capacitor strain sensoraccording to the present invention.

FIG. 13 is a perspective view of a packaging and mounting configurationfor the inter-digitated capacitor of the present invention.

FIG. 14A through FIG. 14H is a flow diagram showing an embodiment of apackaging process for the inter-digitated capacitor and microstripantenna according to the present invention.

FIG. 15 is a perspective view of an embodiment of a spinal plate with anattached inter-digitated strain sensor according to the presentinvention.

FIG. 16 is a flow diagram showing an embodiment of a process forapplying a parylene sealant to a packaged antenna, inter-digitatedcapacitor strain sensor, and associated circuitry according to thepresent invention.

FIG. 17 is a graph illustrating expected strain decrease and plateauresulting from progression of spinal fusion.

FIG. 18 is a schematic diagram of a digital telemetry and calibrationcircuit for use with an inter-digitated capacitor strain sensor systemaccording to the present invention.

FIG. 19 is a graph illustrating an example of expected strain dataoutput of an inter-digitated capacitor strain sensor system according tothe present invention as a function to time during spinal fusion.

FIG. 20 is a perspective view of an embodiment of a blood chemicalmonitor employing an inter-digitated capacitor strain sensor accordingto the present invention.

FIG. 21 is a perspective view of an embodiment of a sealed-chamber heartrate monitor employing an inter-digitated capacitor strain sensoraccording to the present invention.

FIG. 22 is a perspective view of a heart rate monitor blood vessel cuffemploying an inter-digitated capacitor strain sensor according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the system, apparatus, devices andmethods generally shown in FIG. 1 through FIG. 22.

In general terms, the present invention is embodied in a system thatemploys capacitive inter-digitated strain sensing and RF signaltransmission using integrated microfabricated circuitry. The presentinvention generally comprises an implantable capacitive strain sensorthat can produce a reliable, reproducible signal that will indicate viaa radio telemetry signal when strain has changed. An embodiment of thesystem includes an internal power supply subsystem that is configuredfor inductive coupling to an external power source so that batteries arenot required. A further embodiment of the system includes a receiversubsystem to which sensed data is transmitted and collected. Additionalembodiments include variations of the foregoing.

The present invention will be described herein with reference todetecting spinal fusion. It will be appreciated, however, that practiceof the invention is not limited to detecting spinal fusion. For example,the invention can be applied to measuring or monitoring strain invirtually any object, but is ideally suited to strain detection insidethe body of a human or animal. According to other aspects of theinvention, strain monitoring is used as an indicator of medicalconditions including monitoring the progress of spinal fusion,monitoring glucose levels, measuring spinal loading, and monitoringheart rate, which will also be described herein. Therefore, thefollowing description of the invention should be considered asnon-limiting and provided by way of examples.

In one mode of operation, the present invention provides an electronicsolution for detecting spinal fusion more rapidly than through the useof radiographs, and is based on the premise that the spinal fixationinstrumentation used will not be rigid when initially implanted. Forexample, there will be minor gaps between the pedicle screws and thespinal plate that will allow for some movement. The screws will alsomove slightly until bone grows into the threads to hold them rigidlyfixed. The anterior sides of the vertebrae are not fixed, and becausethe two vertebrae are separated by the cushioning intervertebral disc,there will always be some movement from this source. Therefore, thespinal plate anchored to the two pedicle screws will act like a beamwith a moment applied at both ends. The moment will induce bending inthe spinal plate that can be measured as a strain, especially if thespinal plate is necked down to provide a concentrated bending moment atthe center of the plate. Initially, the strain on the spinal place willbe large, but will decrease over time as the bone growth providesadditional fixation. After some period of time, the strain will minimizeat a lower value and remain relatively constant. By periodicallysampling the strain electronically, a curve can be generated, showingthe onset of rigid fixation.

To address the need to detect spinal fusion more rapidly, the inventioncomprises an electronic solution for detecting spinal fusion. The strainsensor and associated circuitry can be bonded directly to the spinalfixation device, which will share load with the bone. Thus, as the spineheals, the implant strain will diminish. There is a time dependentrelationship between strain and fusion that can be detected by measuringstrain in the spinal instrumentation. If spinal fusion can be detectedby a radio telemetry system much earlier than a traditional radiograph,then time spent in bracing or modified activities for spine surgerypatients can be minimized. Accordingly, an aspect of the invention is toreduce the amount of time patients must remain in a brace in order toavoid other complications, such as disuse atrophy, and that thepatients' recovery and eventual outcome is thus improved.

In order to facilitate implantation of the implantable portions of thesystem, the circuitry can be integrated into the fixation plate orencapsulated and attached to the fixation plate. In this way, spinalfixation hardware will contain a strain sensor. Preferably, theimplantable portion of the system is inductively powered by radiofrequency to avoid the complications of implanting batteries withinhumans. Otherwise, batteries will be mounted subcutaneously and removedonce fusion has been determined.

System Overview

Referring first to FIG. 1, a strain measurement/monitoring system 10according to the present invention is schematically illustrated. In theembodiment shown, the system comprises a sensor subsystem 12, aradiofrequency (RF) transmitter subsystem 14 and associated antenna 16,an inductive power subsystem 18, and a receiver subsystem 20 andassociated antenna 22.

The technology for the receiver subsystem 20 and associated antenna 22is commercially available, such as that used for RF ID applications.Such equipment will receive telemetry date as well as provide aninductively coupled power supply. It may, however, be necessary tomodify the operating frequency of the equipment to match the desiredoperation frequency of the sensor system. Therefore, the followingdescription will focus primarily on sensor subsystem 12, RF transmittersubsystem 14 and antenna 16, and inductive power subsystem 18.

RF Transmitter Subsystem

In a preferred embodiment, the RF transmitter subsystem 14 comprisescomponents that will receive the signal from the sensing subsystem 12and use that signal to modulate the frequency (FM) of the carrier signalin order to transmit an output signal through the antenna 16. Referringalso to FIG. 2, in the embodiment shown, the RF transmitter subsystemcomprises a voltage controlled oscillator (VCO) 24 and a power amplifier26 coupled to antenna 16. The power to drive this subsystem is suppliedby an inductive power subsystem 18.

Frequency Selection

A factor in the overall design of the RF transmitter subsystem is thefrequency at which the carrier signal will be transmitted. The operatingfrequency directly affects the dimensions of components in the RFtransmitter subsystem, such as antenna 16. For the purpose of a designfor use as a system implanted in the body of a human or animal, we chosea frequency of 100 GHz although other frequencies could be used. Higherfrequencies tend to have better propagation characteristics and largeravailable bandwidth than lower frequencies, and allow for the use of asmall antenna. Note, however, that safety limits of using such highfrequency in a human body tends to restrict the electric field strengthto approximately 61.4 V/m, the magnetic field strength to approximately0.163 Nm, the power density to approximately 50 W/m², and the durationof exposure to less than approximately 6 minutes.

Modulation Type

Other potential safety concerns are related to the type of RF signalmodulation scheme employed. There are generally three types of signalmodulation; frequency modulation (FM), amplitude modulation (AM), andpulse modulation (PM). PM tends to cause the greatest damage tobiological tissues due to high energy release during short time periods.On the other hand, AM is more susceptible to noise while beingtransmitted through biological tissue, thus leading to potential falsedata readings. Therefore, the amplitude is affected more than frequency.Consequently, frequency modulation (FM) is preferred because itsamplitude is not relevant to the data transmitted and is safer than PM.An additional advantage of FM is the ability to have greater noiseimmunity at greater bandwidths. In summary, a frequency modulated signaltransmitted in a wideband scheme is preferred.

Transmitter

Transmitter selection is also based on several factors. For example, thetransmitter should be suitable for the modulating format selected aswell as suitable for producing the required transmit power to provide areliable link with the receiver subsystem. In the embodiment illustratedin FIG. 1 and FIG. 2, the transmitter comprises a voltage controlledoscillator 24 followed by a power amplifier 26. The modulator in thisconfiguration is VCO 24 being driven by a modulating signal from thesensor subsystem 12.

Voltage Controlled Oscillator

It will be appreciated that the voltage controlled oscillator is animportant component of the RF transmitter subsystem. VCOs in the GHZrange are typically fabricated using standard IC technology and arecurrently integratable. The objective of the VCO is to use an AC signalfrom the sensor subsystem 12 to modulate the signal of the VCO or thecarrier signal. With zero input to the VCO, the VCO will produce a puresinusoidal wave form with a fixed amplitude and frequency. When the VCOreceives an input, it locks itself in a phase locked loop (PLL) toproduce a signal that is modulated in relation to the modulating sensorsignal.

The VCO preferably has a high tuning sensitivity (change in outputfrequency per unit change in the control voltage, Hz/V) that allows formaximizing the modulation of the carrier for an improved signal. Inaddition, power supply pulling (sensitivity of the output frequency tochanges in the power supply voltage, Hz/V) should remain unchanged forimproved gain. This type of behavior is expected during the power up ofthe circuit due to the transit behavior during this time. Therefore,initial readings may not be as accurate, and for this reason sufficienttime should be allowed for each reading. Since the sensor is expected toproduce very little change over the period of measurement, a consistentoutput from the system is a good indication of bypassing the transittime.

There are generally three types of voltage controlled oscillators thatcan be fabricated in integrated circuit form: ring, relaxation, andtuned oscillators. The first two are easier to implement in the presentinvention because they are monolithic and of small size when comparedwith tuned oscillators.

In the present invention, a ring oscillator is preferred. It will beappreciated that the general expression for carrier frequency is:

e(t)=A*Sin(ωc*t+D*Sin(ωm*t)   (1)

where,

$D = \frac{2*\pi*f}{\omega \; m}$

is deviation ratio or the modulating index, ωm=maximum sensor frequency,ωc=center frequency of the carrier, and A=Amplitude of the signal. Theabove equation indicates that a change in the amplitude of ωm changesthe frequency of the carrier. FIG. 3 illustrates the expected responseof a VCO when an input signal is provided. Note that the amplitude ofthe signal is constant while the frequency of the modulated signal isdifferent than the pure sinusoidal wave of the VCO with zero input. Thistype of oscillator response is desired to affectively transmit theinformation with the least amount of noise from the medium.

Antenna

It will be appreciated that the antenna is an important component in anywireless communication system since it is the interface between theelectronics inside the system and the outside world. It is well knownthat, as frequency increases, antenna size generally decreases.Microstrip technology provides a class of antennas that can beintegrated onto the present invention quite easily, and is preferred dueto due to the acceptable performance and the simple manufacturingprocess that is involved. Also, the shape of a microstrip antenna can bevaried based on the needed radiation pattern. Square antennas producegood radiation characteristics and are widely used. An additionaladvantage of such antennas is their conformal configuration which can beplaced on any metallic surface, planer or non-planar, which includes aspinal plate.

Referring to FIG. 4 through FIG. 6, the microstrip antenna 16 employedin the present invention comprises a very thin metallic strip (patch) 30above a conducting ground plane 32, separated by a low-loss dielectricsubstrate 34.

A feed line 36 is also connected to the antenna. In this regard, thereare several feed mechanisms that are available. The location of thefeedline with respect to the antenna patch is determined by theradiation characteristics of the antenna. The simplest and preferredfeed type is the microstrip feedline, an embodiment of which isillustrated in FIG. 4 through FIG. 6. In this embodiment, the feedlineis in the same plane as the antenna and provides for a high degree ofcompactness and efficiency. Feedlines are important because theytransfer the signal to the antenna. The impedance, ratio of voltage tocurrent, of the feedline is the characteristic value and this determinesthe power loss during the transfer. It will also be appreciated that itis important to select a feed mechanism with minimal power loss.

As mentioned above, the preferred embodiment of the RF subsystemoperates at a frequency of 100 GHz. This frequency is inversely relatedto the dimensions of the antenna as described by the following set ofequations:

$\begin{matrix}{{{Height}\mspace{14mu} {of}\mspace{14mu} {substrate}\mspace{34mu} 0.003\lambda} < h < {0.05\lambda}} & (2) \\{{{Width}\mspace{14mu} {of}\mspace{14mu} {antenna}\mspace{14mu} {patch}\mspace{20mu} W} = {\frac{C}{2f_{0}}\left( \frac{{ɛ\; r} + 1}{2} \right)^{{- 1}/2}}} & (3) \\{{{Length}\mspace{14mu} {of}\mspace{14mu} {antenna}\mspace{14mu} {patch}\mspace{20mu} L} = {\frac{C}{2f_{0}\sqrt{ɛ\; e}} - {2\Delta \; L}}} & (4) \\{{ɛ\; e} = {\frac{{ɛ\; r} + 1}{2} + {\frac{{ɛ\; r} - 1}{2}\left( \frac{1 + {12\; h}}{W} \right)^{{- 1}/2}}}} & (5) \\{{\Delta \; L} = {0.412h\frac{\left( {{ɛ\; e} + 0.3} \right)*\left( {\frac{W}{h} + 0.264} \right)}{\left( {{ɛ\; e} - 0.258} \right)\left( {\frac{W}{h} + 0.8} \right)}}} & (6)\end{matrix}$

where C=speed of light; εr=dielectric of substrate; εe=equivalentdielectric, and ΔL=change in length due to fringing fields at the ends.

Using the following Matlab algorithm to simulate the calculations, thedimensions for four antennas shown in Table 1 were calculated:

INDEX I: Matlab Script for the Microstrip Antenna Dimensions:

************************************************************ % programto calculate the dimensions for a rectangular microstrip % antenna witha known thickness % all dimensions in meters or SI units % useassumptions from book: RF MEMS & the Humberto article on TTL  microstrip antenna************************************************************ c=3e8; %speed of light f0= input(‘ frequency of operation= ’); % frequency ofoperation h= input(‘height of dielectric substrate= ’); % thickness ofthe dielectric substrate ebs_r= input(‘ Dielectric of material= ’); %dielectric constant w= c/(2*f0)*(((ebs_r+1)/2){circumflex over ( )}−.5); % width of the patch ebs_e= (ebs_r+1)/2 + (ebs_r−1)/2*(((1+12*h)/w){circumflex over ( )}−.5); % calculate effective K   Num =(ebs_e+0.3)* ( w/h +0.264); % numerator of delta L,   a dummy variable  Den = (ebs_e − 0.258)*(w/h +0.8); % Denominator of delta L,   a dummyvariable    deltaL= 0.412*h * (Num/Den) ;l=(c/(2*f0*(sqrt(ebs_e)))−2*deltaL );% length of the patch w_mm =w*1000% dimensions in mm l_mm=l*1000 % dimensions in mm h_mm=h*1000 %dimensions in mm***********************************************************

A preferred set of dimensions are those in row 3; namely, 0.15 mm×0.588mm×0.463 mm. A dielectric material with a value of ε=12 (such assilicon) is suitable for the substrate. The thickness of the antennapatch is not critical to the performance, and a thickness of 10 μm waschosen for this embodiment.

Note that in selecting a high dielectric constant, the substrate becomeselectrically thick at higher frequencies; namely, λm/4 at 100 GHz, whereAm is the wavelength inside the substrate. Higher thickness leads toincreased surface waves and hence losses. Therefore, in the presentinvention, artificial methods are preferably used to reduce theeffective dielectric constant of the substrate below the antenna. Onemethod to do so is by micromachining the substrate and creating a cavity38 under the antenna as shown in FIG. 5 and FIG. 6. The resultingsubstrate then comprises a composite of air and Si. This configurationcan be achieved by, for example, a DRIE etching of the substrate underthe antenna patch. Assuming a vertical wall etching, this methodimproves the antenna bandwidth and the efficiency over conventionalsubstrates by as much as 60% and 28%, respectively. Preferably, thecavity is designed to have a resonant frequency close to that of theantenna patch to reduce losses.

When the cavity is added, the new effective dielectric constant isestimated to be:

$\begin{matrix}{{ɛ\; r},{{eff} = {\frac{ɛ_{cavity}}{L + {2\Delta \; L}}\left( {L + {2\Delta \; L\; \frac{ɛ_{fringe}}{ɛ_{cavity}}}} \right)}}} & (7)\end{matrix}$

where the dielectric constant for the fringing field region and themixed substrate cavity region are given by:

$\begin{matrix}{\frac{ɛ_{fringe}}{ɛ_{cavity}} = \frac{ɛ_{air} + {\left( {ɛ_{sub} - ɛ_{air}} \right)x_{air}}}{ɛ_{air} + {\left( {ɛ_{sub} - ɛ_{air}} \right)x_{fringe}}}} & (8) \\{ɛ_{cavity} = \frac{ɛ_{air}ɛ_{sub}}{ɛ_{air} + {\left( {ɛ_{sub} - ɛ_{air}} \right)x_{air}}}} & (9)\end{matrix}$

where x_(air) and x_(fringe) are ratios of air to substrate thickness inthe mixed and fringing field regions. Note that with this cavityredesign, the dimensions of the antenna will change based on the newvalues of the dielectric constant of the mixed substrate region.

Analysis of the antenna radiation spectrum is then used to calculate theoptimum location for receiving data from the system. The radiationpattern describes the angular variation of power density of the signalthroughout space. The antenna radiation pattern is divided into twomajor regions; the far field and the near field. It is preferable toreceive the waveform in the far field region to maximize the receivedsignal and to ensure accurate readings.

Referring now to FIG. 7, in a preferred embodiment the microstripantenna is fabricated in a similar manner to a parallel plate capacitoraccording to the steps shown. Top views appear on the left andcross-sectional views appear on the right.

The process begins at step 100 with a silicon wafer.

At step 102, the surface is passivated with thermal oxidation (2 μm).

At step 104, photoresist (PR) is spun onto the passivated surface.

At step 106, a first mask is used to pattern the antenna.

At step 108, aluminum is deposited using LPCVD.

At step 110, excess aluminum is lifted off using acetone.

At step 112, the silicon is etched from the backside using DRIE. Thebottom side of the wafer is also polished down. This creates thedimensions of the air cavity.

At step 114, photoresist is spun onto the backside to create the aircavity.

At step 116, a second mask is used to pattern the air cavity. The PR isthen exposed and developed.

At step 118, silicon is etched from the backside using RIE.

At step 120, SiO₂ is etched from the backside using BOE.

At step 122, the PR is removed using acetone.

At step 124, a second silicon wafer is fusion bonded to the etched waferto create the air cavity.

At step 126, silicon is etched from the backside using DRIE and thebottom side of the wafer is polished down.

At step 128, SiO₂ is deposited on the backside to passivate the surface.

At step 130, titanium is sputtered on the backside to create the groundplane.

At step 132, SiO₂ is deposited on the backside for electrical isolation.

At step 134, gold bumps are patterned for “gold bump compressionbonding.”

Inductive Power Subsystem

It will also be appreciated that supplying power to a sensor for short-or long-term monitoring in human recipients can be a challenge. Agenerally unacceptable method of providing power would consist of havingelectrodes riveted to the patient's skin to connect the microsystem tothe outside world for power and data collection. This“Frankenstein-like” solution could potentially work, but the risk ofinfection and injury to the patient makes this method of supplying powerto the device less than appealing. Furthermore, due to the harshenvironment inside the human body, the sensor cannot contain any toxicmaterials because in the event that the packaging was to fail,contamination of the biological host or damage to the microsensor systemitself could occur. This constraint, along with a relatively shortlifetime, eliminates the possibility of incorporating a chemical batteryinto the system.

Therefore, a more practical method is to supply power through a wirelessmedium. In the embodiment shown, magnetic coupling is employed for thispurpose. The basic approach for supplying power magnetically is toinduce a voltage onto a coil implanted into the biological host with thesensor. This is accomplished by exciting an external coil that islocated directly over the implant, preferably using a conventionalsinusoidal voltage supply (not shown) for excitation. The excitationsignal passes through the body and into the internal coil where analternating current (AC) voltage is induced. The induced voltage in theimplanted coil is then rectified and filtered to create a direct current(DC) source for powering the sensor and associated RF transmittercircuitry. The method of magnetic coupling described above is atechnology known as passive telemetry, or alternatively absorptiontelemetry.

FIG. 8 illustrates various components of an inductive power subsystem 18suitable for use in the present invention. This embodiment is shown inthe context of receiving power from an outer inductive coil 50 whichwould be connected to the external sinusoidal power source, such asprovided by an RF ID type receiver unit 20. In the embodiment shown inFIG. 8, the inductive power subsystem comprises an inner inductive coil52, a rectifier 54, and a regulator 56. As can be seen, the internalinductive power unit essentially comprises a tuned LC receiver, formedby internal coil 52 and a capacitor 58, rectifier 54 and voltageregulator 56. It will be appreciated that the efficiency of powertransmission is related to the degree of coupling of the outer coil 50and the inner coil 52. Factors that affect this efficiency includeshielding of the inner coil, the distance between the coils, and theorientation of the coils with respect to each other.

To improve the power link between the internal and external coils, thesize of the two coils should be optimized for increased couplingcoefficient. Other improvements may be made to the LC circuit of eachcoil if desired. To maintain a constant supply of DC voltage, aregulator followed by a rectifier are used as shown. Also, the internalcoil can be macromachined instead of micromachined to increaseefficiency. In addition, the inductive power subsystem is preferablyhermetically sealed for protection and, to minimize the size of theinternal power unit, the coil is placed outside the hermetically sealedunit and will be located on the implant itself. A wire connection thenwould be used between the inductive power subsystem and the sensor andthe RF transmitter subsystem, and any other related circuitry.

Sensor Subsystem

Two sensing methods that were considered for measuring spinal fusionwere (1) semiconductor-based resistive strain gages and (2) capacitivesensors. With resistive strain gages, minute changes in resistance aredetectable. However, resistive strain gages have several drawbacks whichmake their use in sensing spinal fusion generally undesirable. First,looking at the power consumption of a resistive strain gage, there canbe a fair amount of power dissipation due to the resistance (e.g., powerdissipated=I²*R). Second, resistive strain gages are temperaturedependent devices and could provide unreliable measurements.

Therefore, in the embodiment shown, the sensor comprises a capacitivesensor. A capacitive sensor takes advantage of the absence oftemperature dependence and the minimized power consumption. Becausecapacitive sensors are also generally known to be more sensitive thanresistive sensors, a design based on a change in capacitance due to achange in strain should provide a more accurate measurement.

Two types of capacitive sensors that were considered for this purposewere (1) spacing variation motion sensors and (2) area variation motionsensors. In a spacing variation motion sensor, the change in capacitanceis dependent on the spacing between the two conducting plates. However,a nonlinear relationship exists between the spacing and capacitancechange which presents a problem with measuring capacitance directly. Onthe other hand, in an area variation motion sensor, the change incapacitance is dependent on the area of overlap between the twoconducting plates and capacitance and motion are linearly related. Forpurposes of sensing spinal fusion, an area variation motion sensor ispreferred since capacitance and motion are linearly related andcapacitance can be measured directly.

Note, however, that with a conventional area variation sensor, thecapacitance change due to 25 μm of strain is on the scale of 10⁻¹⁶ F andtherefore, is effectively too small of a change to measure accurately.Referring to FIG. 9, to solve this problem we designed an interdigitatedcapacitor 60 using fifty-one free-standing, inter-digitated fingers.This design yielded fifty parallel plate capacitors adding to the totalcapacitance measurement. As a result, we are able to sense a capacitanceon the order of 10⁻¹⁴ F. Our capacitance sensing relies on the lateralmovement of the inter-digitated fingers. The change in area between thefingers due to the lateral movement results in a change of capacitance.The basic equation for parallel-plate capacitance is

C=[ε _(o)ε_(r) *A*(n−1)]/d=[(ε_(o)ε_(r) *W*l*(n−1)]/d   (10)

where ε_(o)=8.85×10⁻¹² F/m is the permittivity of free space, ε_(r)=1 isthe permittivity of air, “A” is the area of the plates, “d” is the gapbetween the plates, “W” is the width of the plates, “l” is the length ofthe plates, and “n” is the number of inter-digitated fingers.

Before spinal fusion occurs, there will be a strain induced from thebending of the vertebrae. By placing our capacitance sensor in thedirection of this strain, a lateral movement of the plates will causethe area of the capacitor to change (FIG. 9A, FIG. 9B). The capacitancecan then be calculated using

C=[ε_(o)ε_(r) *w*(l−x)*(n−1)]/d   (11)

where “x” is the amount of displacement due to the strain (25 μm).

In addition to the capacitance created between each of the fingers,there is also a capacitance created between the tip of each finger andthe opposing base. This capacitance changes as the gap between thefinger and the base varies. Because we desire our overall capacitance tobe a function of only the lateral variation capacitance between thefingers, we designed our sensor to minimize any capacitances that mightresult from tip of each finger and the opposite base. To minimize thiscapacitance, we designed the gap between the finger and the opposingbase to be large enough that the overall capacitance is not affected bythese space variation capacitances. As a result, we designed the gap,d₂, to be 50 μm.

We also chose our capacitor dimensions such that maximum displacementdue to strain could be sensed. The inter-digitated fingers remain freestanding and function properly as long as the length of each finger doesnot exceed approximately 200 μm. For this reason, we chose the totallength of each finger, L_(Total), to be 200 μm. Since we alreadydeclared the gaps, d₂, on each finger to be 50 μm long, the remaining150 μm was assigned to the capacitance length, L_(Cap) (FIG. 9B). Byusing the relationship between the capacitance, length, width, and gap,a large length and large width and a small gap are desired for a largecapacitance value. Consequently, we chose a length “L_(cap)” of 150 μm,width “W_(Cap)” of 20 μm, a gap “d₁” of 5 μm, and a height “h” of 20 μm.Using 51 inter-digitated fingers, our resulting capacitance under nostrain is

C _(NoStrain)=[(ε_(o)ε_(r) *W*l*(n−1)]/d=[(8.85×10⁻¹²)*(1)*(5×10⁻⁶)*(150×10⁻⁶)*(50)]/(10×10⁻⁶)=C _(NoStrain)=3.34×10⁻¹⁴ F   (12)

During a maximum strain of 25 μm, we found the capacitance to be

C _(25 μm)=[(ε_(o)ε_(r) *w*l*(n−1)]/d=[(8.85×10⁻¹²)*(1)*(5×10⁻⁶)*(150×10⁻⁶-25×10⁻⁶*(50)]/(10×10⁻⁶) C _(25 μm)=2.60×10⁻¹⁴ F   (13)

By carefully choosing these dimensions, we expect to maximize thecapacitance measurement of our sensor.

Since capacitance and motion are linearly related for an area variationsensor, the capacitance can be measured directly. However, a transduceris still needed to translate the capacitance change response to anelectrical output signal. In our case, we prefer a capacitance bridge toconvert the capacitance changes into an electrical voltage output.Capacitance bridges are commonly used as transducers, which is primarilyreason we chose to use it in our design.

Referring to FIG. 10, we designed our capacitance bridge 62 such that wecombined the capacitances of all fifty capacitors as one of the fourlegs, C_(T), of the capacitance bridge. The other three legs (C_(ref1),C_(ref2), and C_(ref3)) are set as reference capacitors. The referencecapacitors are equal to C_(T) during no strain:

C _(ref1) =C _(ref2) =C _(ref3) =C _(T) _(—) _(No Strain)=3.31×10⁻¹⁴ F  (14)

When there spinal fusion has occurred, and therefore, no strain ispresent, we expect the voltage output at V_(sense+) to equal the voltageoutput at V_(sense−).

$\begin{matrix}\begin{matrix}{V_{{sense} +} = {\frac{C_{{ref}\; 1}*\left( {2\mspace{11mu} {Vm}} \right)}{C_{{ref}\; 1} + C_{{ref}\; 3}} - {Vm}}} \\{= {\frac{C_{{ref}\; 1}*\left( {2\mspace{11mu} {Vm}} \right)}{C_{{ref}\; 1} + C_{{ref}\; 3}} - \frac{\left( {C_{{ref}\; 1} + C_{{ref}\; 3}} \right)*{Vm}}{C_{{ref}\; 1} + C_{{ref}\; 3}}}} \\{= \frac{\left( {C_{{ref}\; 1} - C_{{ref}\; 2}} \right)*{Vm}}{C_{{ref}\; 1} + C_{{ref}\; 3}}}\end{matrix} & (15) \\\begin{matrix}{V_{{sense} -} = {\frac{C_{{ref}\; 2}*\left( {2\mspace{11mu} {Vm}} \right)}{C_{{ref}\; 2} + C_{T}} - {Vm}}} \\{= {\frac{C_{{ref}\; 2}*\left( {2\mspace{11mu} {Vm}} \right)}{C_{{ref}\; 2} + C_{T}} - \frac{\left( {C_{{ref}\; 2} + C_{T}} \right)*{Vm}}{C_{{ref}\; 2} + C_{T}}}} \\{= \frac{\left( {C_{{ref}\; 2} - C_{T}} \right)*{Vm}}{C_{{ref}\; 2} + C_{T}}}\end{matrix} & (16)\end{matrix}$

However, before spinal fusion has occurred, strain will be induced dueto the bending of the vertebrate. During this time, we expect to see adifference between the output voltages, V_(sense+) and V_(sense−). Inorder to measure the voltage difference, a differential amplifier 64with a single-ended output as shown in FIG. 11 can be used. Adifferential amplifier is a class of amplifiers that processes thedifference between two signals. Our design includes unity gain bufferson each of the voltage outputs, V_(sense+) and V_(sense−), in order toisolate the nodes from other electronics. The signals are then eachconnected to the differential amplifier which in turn processes thedifference between the two signals. Finally, the transducer outputvoltage, V_(o), can be calculated using:

$\begin{matrix}{V_{o} = {{a_{c\; m}*\frac{V_{{sense} +} + V_{{sense} -}}{2}} - {a_{d\; m}*\frac{V_{{sense} +} - V_{{sense} -}}{2}}}} & (17)\end{matrix}$

where a_(cm) and a_(dm) are the amplifying gains.

In a preferred embodiment, the fabrication of our sensor involvesforty-nine steps and nine masks. The inter-digitated fingers are formedusing poly-silicon and released by wet etching sacrificial phososilicateglass (PSG). This process is shown in FIG. 12 where top plan views areshown on the left and cross-sectional views are shown on the right.

At step 200 the process begins with a silicon wafer.

At step 202, the surface of the wafer is passivated by thermal oxidation(2 μm).

At step 204, polysilicon (0.5 μm) is LPCVD deposited to createbreak-away tethers for the final release of the structure.

At step 206, photoresist (PR) is spun on the surface.

At step 208, a first mask is used to pattern the polysilicon break-waytethers. The PR is then exposed and developed.

At step 210, the polysilicon is etched using RIE.

At step 212, the PR is removed using acetone.

At step 214, a sacrificial PSG (1 μm) is deposited, and also providessome degree of planarization.

At step 216, PR is spun on the surface.

At step 218, a second mask is used to make anchor windows between thefirst and second layers of polysilicon. The PR is then exposed anddeveloped.

At step 220, the PSG is partially etched in 10:1 HF to createconnections to the break-away tethers.

At step 222, the PR is removed with acetone.

At step 224, a second layer of polysilicon (2 μm) is LPCVD deposited.

At step 226, PR is spun on the surface.

At step 228, a third mask is used to pattern anchors. The PR is thenexposed and developed.

At step 230, the polysilicon is etched using RIE.

At step 232, the PR is removed using acetone.

At step 234, a second sacrificial PSG (3 μm) is deposited, and alsoprovides some degree of planarization.

At step 236, PR is spun on the surface.

At step 238, a fourth mask is used to make anchor windows between thesecond and third polysilicon layers. The PR is then exposed anddeveloped.

At step 240, the PSG is etched in 10:1 HF.

At step 242, the PR is removed using acetone.

At step 244, a third layer of polysilicon (20 μm) is LPCVD deposited.

At step 246, PR is spun on the surface.

At step 248, a fifth mask is used to pattern inter-digitated fingers.The PR is then exposed and developed.

At step 250, the polysilicon is etched using RIE.

At step 252, the PR is removed using acetone.

At step 254, a sacrificial PSG (22 μm) is deposited, and also providessome degree of planarization.

At step 256, PR is spun on the surface.

At step 258, a sixth mask is used to make anchor windows between thethird and fourth polysilicon layers. The PR is then exposed anddeveloped.

At step 260, the PSG is etched using 10:1 HF.

At step 262, the PR is removed using acetone.

At step 264, a fourth layer of politician (2 μm) is LPCVD deposited.

At step 266, PR is spun on the surface.

At step 268, a seventh mask is used to pattern anchors. The PR is thenexposed and developed.

At step 270, the polysilicon is etched using RIE.

At step 272, the PR is removed using acetone.

At step 274, PR is spun on the surface.

At step 276, an eighth mask is used to make contact holes. The PR isthen exposed and developed.

At step 278, gold is sputtered on the surfaces to create contacts (10μm).

At step 280, the excess gold is lifted off using acetone.

At step 282, photoresist is spun on the surface.

At step 284, a ninth mask is used to pattern gold bumps for “gold bumpcompression bonding.”

At step 286, gold is sputtered to create gold bumps.

At step 288, the excess gold is lifted off using acetone.

At step 290, the structure is released by etching the sacrificial PSG in10:1 HF.

At step 292, the structure is flipped and aligned with a target spinalplate which also has gold bumps.

At step 294, the structure and target are compressed at roomtemperature.

At step 296, the bonded structure is released from the originalsubstrate using the breakaway tethers.

In order to maximize the amount of strain sensed by the sensor, we wantto minimize any adhesive, transitional material used between the sensorand the spinal plate. Such materials would attenuate the strain sensedby the sensor. As a solution, we utilized a technique called “gold bumpcompression bonding”. This technique allows us to place our sensordirectly on the spinal plate by patterning small, gold bumps on thesensor and the target spinal plate, aligning the two surfaces together,compressing them together at room temperature, and releasing the sensorfrom its substrate by severing its fragile, break-away tethers.

The circuitry for the RF devices and antenna serve as a housing toprotect the inter-digitated fingers of the capacitive strain sensor frombeing contacted by sealant which is used to encapsulate the structure.Thus, the capacitive strain sensor is fabricated separately with goldtethers that hold the inter-digitated fingers in their correct position.Upon assembly, the capacitive strain sensor is inverted and pressed intomounting holes on the spinal plate. The tethers are then broken torelease the inter-digitated fingers, thus allowing them to move with thebending of the spinal plate. Over this suspended, isolated capacitivesensor, the other circuitry and antenna are mounted with an open cavityon the underside to provide isolation for the capacitive sensor.Lead-through wires are used to attach the inductive power subsystem tothe surface of the housing and additional lead through wires are used toattach the antenna and RF subsystem to the capacitive sensor. Thehousing is then sealed to the spinal plate using parylene. Parylene ispreferred because it provides a conformal coating that will not bedegraded by moisture.

FIG. 13 illustrates an embodiment of a packaging configuration 70. FIG.14 illustrates a process for packaging the sensor and antenna in thismanner where top plan views are on the left and cross-sectional viewsare on the right.

At step 300, the process begins with a silicon wafer.

At step 302, photoresist (PR) is spun on the backside of the wafer tocreate a cavity for the sensor.

At step 304, a mask is used to pattern the cavity.

At step 306, silicon is etched from the backside using DRIE, and thephotoresist is removed.

At step 308, gold bumps are patterned for “gold bump compressionbonding.”

At step 310, the antenna is aligned and compressed to the top of thewafer.

At step 312, the structure is aligned with the spinal plate surfacewhich also has gold bumps patterned on the surface.

At step 314, the structure is compressed onto the spinal plate at roomtemperature.

Note that the antenna is specifically designed to be small enough to fitwithin the form factor of the spinal plate and is on the same surface asthe circuitry on the surface of the housing, and is the most externalsurface of the implanted device. It communicates via radio telemetrythrough the tissues and skin with a handheld RF reader. These readersare readily available through the well-established RFID tag market. Toaccommodate our 100 GHz frequency to limit the size of the antenna, acommercially available RF reader would require modification to sensehigher frequencies. Alternatively, we would have to increase the size ofour antenna which would require incorporating the antenna into thespinal instrumentation and use a wire coil rather than the surfacemicromachined patch antenna design described herein.

Spinal fixation plates and related hardware are well known in the art,and not described herein in any particular detail. However, there arecertain features of the sensor system described herein that makemodifications of a standard spinal plate worth consideration.

For example, it is desirable to fabricate the spinal plate, nuts, andpedicle screws from titanium. Titanium is nontoxic, hypoallergenic,biocompatible and exceptionally corrosion resistant. Titanium is also anonmagnetic material. Since our sensors and circuitry will be mounted onthe spinal plate, electromagnetic interference will be minimized byusing titanium.

FIG. 15 illustrates a machined titanium spinal plate 80 with the sensorsystem described above attached. Note that, using the equations forbending in a beam, rigidly fixed at both ends and with an appliedbending moment at both ends from the pedicle screws, we can obtain astrain of 25 μm with the sensor system. This is a very small signal, butwell within the capabilities of our capacitive strain sensor. Although aportion of the beam shown in FIG. 15 has been machined down to create aconcentrated bending location 82, the overall strength of the beam hasnot been compromised. The strength of the titanium plate has a generoussafety factor, and actually only needs to remain intact until the bonyingrowth has finished. Long term, the strength of the fusion is providedentirely by new bone and not by the implanted hardware.

As indicated above, the system components can be sealed with a materialsuch as parylene. This sealant can be applied, for example, using aprocess which is illustrated in flow chart form in FIG. 16.

The electronic strain measurement system described herein is designed toreplace clinical x-rays, as they are often inconclusive until the bonehas completely mineralized. During surgery, one of the spinal plateswould be replaced with an electronically instrumented spinal plate. Forhumans, the goal would be to have all the instrumentation fitted onto anotherwise unaltered spinal plate.

In use for detecting the progress of spinal fusion, a handheld receiverunit would be brought into proximity of the sensor and an initial strainlevel would be recorded. Then, once a week during routine office visits,the handheld sensor would again be brought into proximity of the sensorto get additional strain level recordings. As shown in FIG. 17, overtime, the level of strain should decrease and eventually plateau at alower level. This should occur within eight to twelve weeks followingsurgery, at which time the fusion can be proclaimed solid and thepatient's external bracing can be removed.

Referring now to FIG. 1, FIG. 10, FIG. 11 and FIG. 18, in one embodimentsensor subsystem 12 includes amplifier 64 as described above, as well asa second amplifier 66 and a 12-bit A/D converter 68. The output of theND converter 68 provides a digital signal to RF subsystem 14. In thisembodiment, the data can be transmitted as digital telemetry associatedwith the RF signal. Packet and other transmission techniques can be usedas well.

In the digital telemetry embodiment illustrated above, prior toimplantation, the inter-digitated capacitor strain sensor is preferablyset to a “neutral” baseline. For example, this may be accomplished usingthe above-described operational amplifiers with a 12-bit A/D resolutionthat creates 4096 databits. The databits would be subdivided so that apercentage of the databits represents an equivalent range of the knownvalues of strain for the area of interest. If, for example, a spinalimplant experiences 1000με (microstrain) when implanted, each 1με wouldcorrespond to approximately 4 databits. Therefore, a neutral baselinevalue would be 2048 databits prior to implantation.

Amplifier 64 should never saturate; if it does, the output data becomesunusable. The system may still output a databit value, but it will be aconstant value, virtually unvarying over the entire measurement period.The external receiver subsystem is preferably configured to detect thisfailure mode. This can be corrected by resetting the neutral baseline toa new value until it is within range.

The second amplifier 66 is employed to set the operating range of thedevice. For example, in spinal implants a normal range of strain isapproximately 100με. The second amplifier 66 is thus preset to represent+/−100με or a range of 200με. In a 12-bit A/D, this corresponds to 1μεchange for every 20 databits of change in the strain.

Note also that, in some applications, the inter-digitated capacitorsensor and implant hardware are subjected to an initial strain by thesurgeon. For example, in spine surgery, the torque applied to thepedicle screws induces approximately 600με in the spinal hardware. Thus,the inter-digitated capacitor sensor is preferably adjustable tore-center its value at the example 2048 databits. As the exact amount ofinduced strain cannot be predetermined, this adjustability is importantfor good performance of the sensor system.

Depending on the application, the strain after insertion or implantationwill either increase, decrease, or oscillate with time and conditions.In a spinal fusion application, the spinal implant hardware mayinitially be in flexion or extension, depending on the completelyvariable orientation of the pedicle screws, plates, rods, or cages. Forexample, databit values below 2048 may indicate extension and valuesabove 2048 may indicate flexion. Thus, the external receiver subsystemthat communicates with the inter-digitated capacitor sensor should beable to analyze the initial change in strain from baseline and deducethe initial orientation of the spinal hardware. This information wouldthen be stored in the external receiver subsystem for use in analgorithm that calculates the actual change in strain. Over time, theoverall strain may decrease towards a plateau value, but the “sign” ofthis change is dependent on the initial orientation.

It will be appreciated that, although the system has been describedabove in the context of detecting spinal fusion, strain can be used asan indicator of other biomedical conditions as well. Using MEMStransduction, the system allows for the implantation or insertion of aninter-digitated capacitor strain sensor into area of interest.Advantageously, the system employs a sensitive inter-digitatedcapacitive strain sensor and RF transmitter subsystem are microscopic insize, temperature-independent, use no batteries, use biocompatiblematerials, are sealed from the environment, and can easily be integratedinto an implant or used as part of a self-contained transponder unit.Preferably, RF frequencies are used which fall within publicly availablebands and which are safe to biological tissues. In essence, the systemcan be considered a lifetime implant.

As described above, the RF transmitter subsystem communicates sensorinformation to an external receiver subsystem, which may, for example,comprise a commercial RF ID tag type receiver. The receiver subsystemmay be embodied in many forms such as a handheld unit, portable unit, orwristwatch-style unit, and even contain data processing capabilities orcapabilities to interface with a computer.

Preferably, algorithmic information used for data processing can bestored, processed, and analyzed externally thus keeping the system smallin size and allowing for use of an inductive power subsystem. In analternative embodiment, the system may include memory or the like forstoring databit information from the sensor. This will provide theoption of recording periodic or random time points for later analysis,and can provide for short-term or long-term storage. A healthcareprofessional, for example, would later query the device with atransmitter and signal it to download its stored data. The device couldoptionally be erased after download for long term studies. Thisconfiguration would require use of an external transceiver forbidirectional communication as an alternative to the receiver subsystempreviously described. In addition, since data would be stored, thisenhanced embodiment of the system would likely require a replaceableand/or rechargeable power source subcutaneous to the skin.

In addition, the external wireless transceiver can be configured toprocess the databit information received if desired. For example, thetransceiver could include a processor and associated software thatsubtracts the databit information from the carrier RF wave, averages thedatabits into a single value, and applies the appropriate algorithms toconvert the databit reading into a useable number. A 1000 Hz transceivergathering strain information for one second would generate 1000 databitsfor each reading. If the averaged value were 2500 databits, the outputstrain would be 12.5με. This would be added to the baseline value ofperhaps 600με and give a reading to the surgeon of 612.5με. Each ofthese averaged values could be recorded over time to show trends in thestrain, such as a slow decline and eventual plateau in spinal strainsuch as illustrated in the example shown in FIG. 19. In someapplications, user education may be needed if the user or patient caninfluence the sensor reading by, for instance, body position. Ifnecessary, an operating protocol might be needed to inform the user howto orient the patient for consistent readings over a long term study.

As indicated above, the system is applicable to detecting biomedicalconditions in general and has far reaching application. For example,referring to FIG. 20, a sensor apparatus 400 is shown that can be usedfor measurement of blood chemicals, factors, and minerals using MEMS. Inthis embodiment, the inter-digitated capacitor sensor would, forexample, be inserted into the forearm with a syringe into forearm, ortethered inside a vein as needed to expose sensor to blood stream. Thesensor would be mounted on a housing 402 that includes a chamber 404containing a hydrogel, hydrophilic polymer, or other biocompatiblematerial 406 that dynamically and reversibly swells when exposed tospecific chemicals. High selectivity would be a crucial characteristicof the hydrogel. For example, glucose and lactate are both found in theblood, and have similar affinities in many hydrogels. It is important tothus test the marker of interest against potential competitive markers.FIG. 20 illustrates one configuration for a blood chemical sensor with adisc of polymer 406 swelling to induce a strain.

Swelling of the hydrogel, polymer or other material would induce astrain in the inter-digitated capacitor sensor that would be transmittedby its corresponding transponder. This strain would correspond to aspecific concentration of specific marker within the blood and bodilyfluids, such as glucose, electrolytes, sodium, hydration level, pH,toxic chemicals, or heavy metals (lead, mercury, chromium, etc). Thus,the strain can inform the user of a high or low level of a specificmarker of interest. This would be of extreme interest to diabetics,endurance athletes, and military personnel in the field.

In this embodiment, an external wireless transceiver, most likely awristwatch-style device, would analyze the strain information andprocess it with an algorithm to display glucose level, electrolytelevels (perhaps several key variables on one unit), etc. in termscommonly used for that application. Potentially, the wristwatch-styletransceiver could communicate with a remote location for monitoring andadvice by a professional, such as the user's physician or a militaryperson's superior officers. The wristwatch style unit could also beconfigured to provide alerts or alarms to tell the user to take aspecific action, such as replenish electrolyte levels, seek immediatemedical attention, or inject insulin.

Similarly, the system could be configured for measurement of heart rate.

Similar to the application described above with reference to FIG. 20, aheart rate monitor inter-digitated capacitor sensor could be simplyinjected by syringe beneath the skin, since the entire body “pulses”upon each beat of the heart. For an injected version, the sensor wouldhave a sealed chamber that would flex under the pressure of each pulseand induce a strain that would be transmitted wirelessly to awristwatch-style unit to give the user continuous heart rate monitoring.FIG. 21 illustrates an embodiment of a heart rate monitor 500 with asealed chamber 502. Alternatively, as illustrated in FIG. 22, aninter-digitated capacitor sensor could be configured as a blood vesselcuff for attachment around a blood vessel 602 of the forearm (or otherdesirable region of the body) for larger strain potentials. If needed toachieve a strain above the background noise of the body, the sensorcould be designed to cuff the external surface of blood vessel. Thiswould induce a hoop stress and would maximize the strain potential fromthe blood vessel.

As described above, the preferred sensor configuration comprises aninter-digitated capacitor sensor. It will be appreciated that othertypes of sensors could be used, but that an inter-digitated capacitorsensor is clearly advantageous. Other types of sensors, althoughinferior to the inter-digitated capacitor sensor, includemicrofabricated pies-resistive strain gages configured in a Wheatstonebridge. In this configuration, change in resistance of the bridge ismonitored as a voltage, converted to a digital signal, and transmittedto a handheld receiver.

Also as described above, in order to create a successful dataacquisition system, several technologies must be integrated togetherthat are driven by the microsensor design. The quantity to be measuredand the environment that the sensor will reside in will determine thetype of sensor, and the packaging needed to protect it from thepotentially harsh surroundings. Although the description above containsmany details, these should not be construed as limiting the scope of theinvention but as merely providing illustrations of some of the presentlypreferred embodiments of this invention. Therefore, it will beappreciated that the scope of the present invention fully encompassesother embodiments which may become obvious to those skilled in the art,and that the scope of the present invention is accordingly to be limitedby nothing other than the appended claims, in which reference to anelement in the singular is not intended to mean “one and only one”unless explicitly so stated, but rather “one or more.” All structural,chemical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the present invention, for it to be encompassedby the present claims. Furthermore, no element, component, or methodstep in the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

TABLE 1 Calculated Dimensions of Microstrip Antenna (m) Dimensions f L hW ε 1 100 GHz 2.29 *10E−4 1*10E−3 9.49*10E−4  4 2 100 GHz 7.98*10E−41.5*10E−4 9.49*10E−4  4 3 100 GHz 4.63*10E−4 1.5*10E−4 5.88*10E−4 12 4100 GHz  .0056 1.5*10E−4  .0059 12

1. An apparatus for sensing strain, comprising: a sensor; said sensor comprising an inter-digitated area variation capacitor; wherein said sensor comprises a plurality of free-standing inter-digitated fingers; and wherein lateral movement of the inter-digitated fingers produces a change in capacitance detected by said sensor; a transmitter; said transmitter coupled to said sensor; and an antenna; said antenna coupled to said transmitter; wherein said sensor, said transmitter, and said antenna are adapted for implantation in a biological host; wherein the sensor is configured to be coupled to an internal body member of the host; wherein said sensor, said transmitter, and said antenna are adapted for monitoring said change in capacitance to measure a characteristic of loading on said body member.
 2. An apparatus as recited in claim 1: wherein said body member comprises a first bone segment.
 3. An apparatus as recited in claim 2: wherein said sensor is adapted for mounting to a fixation device configured to attach to at least said first bone segment and configured to produce a signal representative of strain in said fixation device; and wherein said transmitter is configured for transmitting said signal representative of strain.
 4. An apparatus as recited in claim 3: wherein said first bone segment comprises a first vertebra, and wherein said fixation device comprises a spinal fixation device configured to couple a first vertebra to a second vertebra.
 5. An apparatus as recited in claim 4: wherein said spinal fixation device has a central area; wherein said spinal fixation device is reduced in width near said central area; and wherein said sensor, said transmitter, and said antenna are affixed to said spinal fixation device proximate to where said spinal fixation device is reduced in width.
 6. An apparatus for sensing strain, comprising: a sensor; said sensor comprising an inter-digitated area variation capacitor; wherein said sensor comprises a plurality of free-standing inter-digitated fingers; and wherein lateral movement of the inter-digitated fingers produces a change in capacitance detected by said sensor; a transmitter; said transmitter coupled to said sensor; and an antenna; said antenna coupled to said transmitter; wherein said sensor, said transmitter, and said antenna are adapted for implantation in a biological host; and wherein said sensor is adapted for measuring skeletal loading via said change in capacitance.
 7. A system as recited in claim 6: wherein said sensor is adapted for mounting to a fixation device coupled between a first bone segment and a second bone segment; the sensor being configured to produce a signal representative of strain in said fixation device; and wherein said transmitter is configured for transmitting said signal representative of strain.
 8. A system as recited in claim 6: wherein said fixation device has a central area; wherein said fixation device is reduced in width near said central area; and wherein said sensor, said transmitter, and said antenna are affixed to said fixation device proximate to where said fixation device is reduced in width.
 9. An apparatus for sensing strain, comprising: a sensor; said sensor comprising an inter-digitated area variation capacitor; a transmitter; said transmitter coupled to said sensor; and an antenna; said antenna coupled to said transmitter; wherein said sensor, said transmitter, and said antenna are adapted for implantation in a biological host; wherein said sensor is adapted for coupling to a fixation device implanted within the body of the biological host; wherein said sensor is configured to produce a signal representative of strain in said fixation device; and wherein said transmitter is configured for transmitting said signal representative of strain; wherein said inter-digitated area variation capacitor comprises a plurality of free-standing inter-digitated fingers; and wherein lateral movement of the inter-digitated fingers produces a change in capacitance detected by said sensor.
 10. An apparatus as recited in claim 9: wherein said fixation device is configured to attach to a first body member and a second body member.
 11. An apparatus as recited in claim 10: wherein said first body member and a second body member comprise bony segments.
 12. An apparatus as recited in claim 9: wherein said fixation device has a central area; wherein said fixation device is reduced in width near said central area; and wherein said sensor, said transmitter, and said antenna are affixed to said fixation device proximate to where said fixation device is reduced in width.
 13. An apparatus as recited in claim 9, wherein said sensor is encapsulated in a housing such that the sensor is sealed from the environment within the biological host.
 14. An apparatus as recited in claim 13, wherein said transmitter and said antenna form said housing for encapsulating said sensor.
 15. An apparatus as recited in claim 13, wherein said sensor, said transmitter, and said antenna are adapted for permanent implantation in a biological host.
 16. An apparatus as recited in claim 9, wherein said sensor has a sensitivity of 10⁻¹⁴ F.
 17. An apparatus as recited in claim 9: wherein lateral movement of the inter-digitated fingers and said change in capacitance are linearly related.
 18. An apparatus as recited in claim 9, further comprising: a power supply; said power supply configured for inductive coupling to a power source; said power supply coupled to said sensor and said transmitter; wherein said power supply is adapted for implantation in a biological host.
 19. An apparatus as recited in claim 18, wherein said power supply comprises: an inductive coil; a rectifier coupled to said inductive coil; and a regular coupled to said rectifier.
 20. An apparatus as recited in claim 9, further comprising a calibration circuit for calibrating said sensor by adjusting a baseline characteristic of said sensor. 